How do I display the absolute file (or directory) path in the UI
There is no reason to display the absolute path nor convert it or set some default path because the user can set it to absolute or relative in the File Browser (N) which is actually a nice feature. If you really need an absolute path representation at some point, just call bpy.path.abspath(path) as explained in How can I get the full path of the selected file from the file explorer?.

Further reading

How can I get the full path of the selected file from the file explorer? (potential duplicate)
Trouble file StringProperty subtype FILE_PATH (potential duplicate)